The formation of a new species through the development by individuals of a new habitat, without going beyond the range, is called sympatric (sympatric, sympatric) or ecological speciation. Sympatry occurs within the same population.
signs
The factor of ecological speciation is different, separate conditions of existence within the same area. Most often, the reasons for the divergence of species are different ways of feeding or the seasonality of the reproduction of offspring.
Initially, the formation of a new species is influenced by genetic isolation (in fact, a mutation) that occurs within the same population. The accumulated beneficial mutations lead to a change in the gene pool of a part of the population, and then to reproductive isolation.
The main features of the ecological formation of species are:
- dispersal of individuals within the same range under different environmental conditions;
- the formation of mutations that help occupy ecological niches;
- the action of natural selection;
- consolidation and transmission of useful traits by inheritance;
- reproductive isolation.
A striking example of ecological speciation is alfalfa species. The crescent alfalfa grows at the foot of the mountains, and the sticky alfalfa grows in the mountains. Most likely, both species "left" from the same population, but occupied different ecological niches without changing their range.
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Rice. 1. Alfalfa sickle-shaped and sticky.
Another example is the blackbird species. One lives in the forest, the second lives next to a person, but within the same range.
One of the mechanisms of the ecological formation of a species is disruptive or tearing selection. This is a kind of natural selection that simultaneously supports two or more groups of the same population with opposite traits. Often "breaks" the population on the basis of reproduction: some individuals reproduce offspring at one time, some - at another. An example is a large rattle: due to summer mowing, part of the population blooms in spring, part - at the end of summer.
Rice. 2. The rattle is large.
Ways
Sympathy arises in two ways:
- as a result of polyploidy - a multiple increase in the haploid set of chromosomes in cells during meiosis (a type of mutation);
- by hybridization - crossing individuals of different species, resulting in an individual with a unique genotype.
Polyploidy is more common in plants than in animals and is widely used in breeding to increase yields. For example, due to polyploidy, the number of cotton chromosomes increases from 26 to 52, which affects the volume of seeds and cotton productivity.
Cultural plum was obtained during the hybridization of blackthorn and cherry plum. An example of natural hybridization is the mountain ash found in the forests of Siberia.
Rice. 3. Ryabinokizilnik.
The ecological formation of species occurs relatively quickly compared to the geographic (alopatric) mode of speciation. Often sympatry is accompanied by allopatry - territorial isolation.
What have we learned?
Ecological or sympatric formation of species is possible within the same range and within the same species. For various reasons, individuals develop new habitats, gradually developing and fixing adaptations in the genotype. In the course of natural selection, a group of adapted individuals is singled out within one species, which, under the action of reproductive isolation, becomes an independent species.
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A new species may arise from one or a group of contiguous populations located on the periphery of the range of the original species. Such a view is called allopatric (from the Greek allos - different, patris - homeland). A new species may arise within the range of the original species. This speciation pathway is called sympatric (from Greek syn - together, patris - homeland). The third way of speciation is through the gradual change of the same species over time, without any divergence of the original groups. Such a view is called phyletic.
I. Allopatric (geographical) - with a change of homeland (area), i.e. speciation based on geographic isolation (Fig. 3).
Rice. 3. Methods of allopatric speciation
In allopatric speciation, new species can arise through fragmentation or during the dispersal of the original species, during which peripheral populations and their groups, increasingly distant from the center of dispersal, intensively transforming under new conditions, become the ancestors of new species.
1. Fragmentation (disintegration) of the population area into several (as a rule, with spatial isolation).
2. Settlement of the original species from the center of the population - part of the individuals of the population falls into other conditions.
II. sympatric – emergence of a new species within the range (Fig. 4).
It can occur in two main ways: autopolyploidy, allopolyploidy.
1. Autopolyploidy - a rapid change in the karyotype (delayed divergence of chromosomes in meiosis): doubling, tripling, and so on of the main set of chromosomes of ancestral species.
2. Allopolyploidy or hybridogenic speciation. More common in plants - crossing different species, followed by doubling the number of chromosomes.
Cultivated plum = sloe (2n = 32) x cherry plum (2n = 16) followed by chromosome doubling 2n = 48.
A feature of the sympatric pathway of speciation is the emergence of new species morphologically close to the original species. For example, with polyploidy, the size increases, but the appearance of the plants does not change.
Rice. 4. Method of sympatric speciation - autopolyploidy. An example of the emergence of reproductive isolation during polyploidization in plants of Dicanthium annulatum, inhabiting the territory of Hindustan.
III. Phyletic speciation is a gradual slow change in a species over time, i.e. from generation to generation, changes, new signs are acquired. At the same time, there always remains the possibility that at some stage of evolution other groups could diverge from a single phyletic trunk. Therefore, pure phyletic evolution is practically impossible.
Questions for chapter 4:
1. Define the species and its criteria.
2. What is the essence of population thinking in modern biology?
3. Why is the elementary unit of the evolutionary process a population, and not an individual and a species as a whole?
4. Determine the structure of the species and population, their main characteristics.
5. What is the importance of elementary evolutionary factors in evolution?
6. Determine the place of the concept of the struggle for existence among the mechanisms of the evolutionary process.
7. What are the forms of natural selection, what is their difference?
8. What is the contribution of genetics to the development of the theory of natural selection?
9. What is the relationship between the processes of elimination and natural selection?
10. What is the creative role of natural selection?
11. Justify the thesis: "evolution is a process of adaptiogenesis".
12. Ways and methods of speciation?
13. How is allopatric speciation different from sympatric?
14. Is it possible to determine the boundaries of a species in paleontological material during phyletic speciation?
Recognizing the reality of the species, Darwin proved that in nature there is a process speciation- the emergence of new species on the basis of existing ones under the influence of the driving forces of evolution. According to modern ideas about evolution, the formation of a new species occurs within the population - the elementary unit of evolution. Populations are genetically open systems. And as long as there is a flow of genes between them as a result of the migration of individuals, the species remains a single genetically closed system. However, the emergence of isolation (barrier) between two populations leads to the accumulation of hereditary differences in them, which prevent the individuals of these populations from crossing at subsequent meetings. This proves that populations are becoming genetically closed systems and hence new species. This means that the process of speciation has taken place.
Speciation is an evolutionary process of transformation of genetically open systems - populations - into genetically closed systems - new species.
Speciation is a complex and lengthy process that includes intermediate stages and requires the presence of certain factors.
Speciation factors
In populations of one species, the action of the prerequisites of evolution leads to the emergence of a diversity of genotypes and phenotypes. This is the basis for the struggle for existence and natural selection. The action of natural selection on populations whose living conditions are different makes them slightly different. However, the differences between individuals that have arisen as a result of selection will be smoothed out if the individuals of the populations begin to interbreed with each other. In order for the process of speciation to begin at the level of these populations, isolation between them is necessary, which prevents the exchange of genetic information. There are two forms of isolation: geographical and biological.
Geographic (spatial) isolation- isolation of a certain population from another population of the same species by some barriers that are difficult to overcome. The first reason is large territorial gaps between populations in species with mosaic ranges. The occurrence of these gaps may be associated with glaciers, human activities, or the dispersal of populations outside the original range. The second reason is the geographical barriers separating populations (rivers, mountains, gorges, forest areas, meadows, swamps). Geographical isolation prevents individuals from separated populations from interbreeding freely due to the impossibility of their meeting due to a geographical barrier.
biological isolation due to biological differences between individuals of populations. Depending on the nature of the differences, four types of biological isolation are distinguished: ecological, ethological, morphophysiological and genetic.
Environmental isolation due to a shift in reproductive periods (terms of flowering, nesting, mating, spawning) or different breeding sites, which prevents free crossing of individuals in populations.
If populations of herbaceous plants fall into a zone of increased moisture, then their flowering time is shifted compared to other populations. In birds, populations of the same species may differ in terms of nesting and mating, depending on the location of nests in different parts of the tree crown or in the shrub layer.
Ethological isolation due to the peculiarities of the behavior of individuals during the mating season. At first glance, insignificant differences in courtship rituals in the exchange of visual, sound, chemical signals can lead to the termination of this ritual and the restriction of mating.
Morphophysiological isolation due to differences in the size of individuals or in the structure of male copulatory organs (some species of pulmonary mollusks, rodents). It does not interfere with the meeting of the sexes, but prevents the crossing of individuals due to the impossibility of fertilization.
genetic isolation due to large chromosomal and genomic rearrangements that cause differences in the number, shape and composition of chromosomes. It does not interfere with the meeting of the sexes and fertilization. But it excludes the exchange of genetic information between populations due to the death of zygotes after fertilization, varying degrees of sterility of hybrids and their reduced viability.
The effect of any form of isolation on evolutionary material is not directed, but is a necessary condition for enhancing genetic differences between populations. An important characteristic of isolation is its duration, due to which the action of multidirectional natural selection leads to a divergence of the characteristics of populations - divergences. As a result, populations become varieties, or race. Maintaining isolation leads to increased differences between varieties, and they turn into subspecies. If increasing differences between subspecies prevents them from interbreeding, then they have become genetically closed systems. There was a reproductive isolation between them. The subspecies have become new species.
Thus, the factors of speciation are:
- prerequisites for evolution: mutational and combinative variability, population waves, gene flow and drift, isolation;
- driving forces of evolution: struggle for existence, natural selection.
The processes occurring within a species at the population level under the influence of these factors and leading to the formation of new species can be considered as the initial stage of evolution - microevolution.
Further, evolution continues at the level of species, genera, families according to the same mechanism and under the influence of the same prerequisites and driving forces of evolution. This stage of evolution is called macroevolution. Microevolution and macroevolution are stages of a single evolutionary process.
Speciation methods
Depending on the form of isolation of populations, two methods of speciation are distinguished: allopatric and sympatric.
allopatric(from Greek. allos- different, patris- motherland) speciation proceeds in the presence of geographic isolation. Populations of the same species are separated by large distances or geographical barriers. The resulting geographic races and subspecies have ranges that do not overlap with the maternal range. An example of allopatric speciation is the presence of two subspecies of the American squirrel and three subspecies of blue jays. They live in different geographical areas of North America. On the Eurasian continent, there are three subspecies of the great tit, which were formed as a result of geographic isolation. There are also subspecies of sparrows, wrens, woodpeckers, which have different distribution areas.
Sympatric speciation(from Greek. syn- together, patris- homeland) occurs in the presence of biological isolation. Populations of the same species are within the maternal range, but cannot interbreed due to biological differences between their individuals. Sympatric speciation can manifest itself in plants with the specialization of pollinating insects in the pollination of flowers of a certain shape. For example, bees are an isolating factor between races of snapdragon plants. They never move from flying around the flowers of one race to another. Some plants (large rattle, white mari) form seasonal races that differ in terms of flowering. In a number of fish species (herring, perch, carp, etc.), seasonal races coexist with different spawning periods.
Speciation factors are: preconditions and driving forces of evolution. Allocate geographical and biological forms of isolation. Depending on the form of isolation, allopatric or sympatric speciation can occur in nature. Speciation is the result of microevolution.
>> Speciation
Speciation
1. Define the species. What kind of criteria do you know?
2. In what cases can differences between populations arising from changes in living conditions lead to the formation of new species?
Evolutionary changes occurring at the population, intraspecific level are called microevolution. The process of microevolution has two forms: phyletic evolution and speciation. Phyletic evolution means gradual changes occurring over time within a single species, populations or groups of populations. As a rule, as a result of these changes, the adaptability of organisms to the environment increases.
Speciation occurs when a species splits into two or more new species. It is this process that provides a huge variety of the organic world.
Stages of speciation.
Speciation usually consists of two stages: the first is the emergence of reproductive isolation, the second is its consolidation natural selection.
At the first stage of speciation, the exchange of genes between two populations of a given species must be terminated; this usually occurs as a result of geographical separation, such as the emergence of a mountain range between populations, a glacier, a water barrier, etc. The lack of gene exchange between two populations creates the possibility for their genetic divergence (divergence). Such divergence can also arise as a result of adaptation of organisms to local conditions, and as a result of random changes in the composition of the gene pool of each of the populations (see § 56 “Changes in the gene pool of populations”), (As genetic differences accumulate between isolated populations, due to changes in conditions and lifestyle, prezygotic isolating mechanisms occur, such as differences in timing breeding, in behavior, etc.
In the future, the division of populations can be fixed due to the development of postzygotic isolating mechanisms. The division of species becomes irreversible. The process of emergence of complete isolation is supported by natural selection.
Specific pathways for the emergence of new species may be different. In general, there are two main forms of speciation: allopatric and sympatric.
allopatric speciation.
This form of speciation is associated with the expansion of the range of the original species and is carried out in the course of long-term geographical isolation of populations. The emergence of geographical barriers (mountain ranges, sea straits, etc.) leads to the emergence of isolates - geographically isolated populations. As a rule, this occurs at the border of the range of the original species, where the living conditions are somewhat different from the usual ones and the processes of natural selection are actively taking place. As a result, the single gene pool of the species seems to be torn apart.
Any territorial population is characterized by its own gene pool with its characteristic frequencies of occurrence of different alleles. The interruption of the flow of genes between isolates, on the one hand, and the action of natural selection, on the other, ultimately lead to their reproductive isolation and the formation of independent species. Such speciation associated with the spatial separation of populations is also called geographic. The scheme of geographic speciation is shown in Figure 81.
Previously, you have already familiarized yourself with examples of geographical speciation, considering the emergence of modern lily of the valley species from the original species that lived millions of years ago in the broad-leaved forests of Europe. The invasion of the glacier tore the single range of the lily of the valley into several parts. It has been preserved in forest areas that have avoided glaciation: in the Far East, southern Europe, and the Transcaucasus. When the glacier receded, the lily of the valley spread again in Europe, forming a new species, larger, with a wide corolla, and in the Far East - a species with red petioles and a wax coating on the leaves.
Sympatric speciation.
Sympatric speciation is associated with the emergence of a new form within the initial population. Such origin can occur as a result of ecological (for example, food) specialization, therefore this form of speciation is often called ecological. It is believed that five species of tits were formed in connection with food specialization: according to the choice of feeding places, according to the composition of the food eaten, according to the methods of their search and production For example, the great tit hollows branches and tree trunks; small species (blue tit) - only stems of herbaceous plants. The smallest species (moscovites, crested tits) often examine the terminal branches of trees in search of food. The great tit feeds on large insects; blue tit, tit and moskovka get small insects in the cracks of the bark and in the kidneys; the crested tit feeds on the seeds of coniferous trees (Fig. 82).
Another form of sympatric speciation is sudden, occurring as a result of chromosomal mutations, polyploidy and hybridization. It is known that closely related types of potatoes differ from each other by a multiple set of numbers; n = 12, 24, 48, 72. This suggests that the corresponding species were formed as a result of polyploidy, i.e., by multiplying the number of chromosomes of the original ancestral species (Fig. 83). In plants, as a result of polyploidy, isolating mechanisms are able to form during the life of a single generation. A multiple increase in the number of chromosomes within one species can occur spontaneously; or the multiplication of chromosomes occurs as a result of the crossing of closely related organisms.
Sometimes speciation occurs due to hybridization with subsequent doubling of the number of chromosomes; due to the doubling of chromosomes, normal germ cells are formed in such organisms and the development of offspring proceeds without disturbance. Cultural plum with 2n = 48 chromosomes, for example, arose by crossing blackthorn (n - 16) with cherry plum (n = 8), followed by doubling the number of chromosomes.
Thus, the formation of new species as a result of chromosomal rearrangements can occur in populations inhabiting the same geographical area and not separated by any barriers.
Microevolution. Allopatric or geographic speciation. Sympatric (ecological and sudden) speciation.
1. Name the main forms of speciation. Give examples of geographic speciation.
2. What is polyploidy? What role does it play in the formation of species?
3. Which of the plant and animal species known to you arose as a result of chromosomal rearrangements?
Discuss what role in speciation play various isolation mechanisms. What form of selection plays a decisive role in the processes of speciation?
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology Grade 10
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Question 1. List the ways of speciation
Speciation in typical cases consists in the division of an initially single species into two or more new ones. This is due to the emergence of isolation barriers between populations and the deepening of differences between the gene pools of populations under the influence of natural selection up to genetic isolation. Speciation can occur due to the gradual transformation of the original species into a new one or due to the hybridization of two species (for example, blackthorn + cherry plum = plum). However, much more often speciation is associated with the divergence (divergence) of characters in populations. As a result, a species whose range has been divided into several populations (for example, each lives on its own island) may eventually form several new species.
Phshetic speciation is also distinguished. It consists in the gradual transformation over time of one species into another. This method is observed if changes in conditions capture the entire area.
They also distinguish hybridogenic speciation (synthesisogenesis or syngenesis). This type of speciation is common in plants: according to some estimates, more than 50% of plant species are hybrid forms - allopolyploids. For example, the cultural plum with 2n = 48 arose by hybridization of the blackthorn 2n = 32 with the cherry plum 2n = 16, followed by a doubling of the number of chromosomes. Some species of pikulnik, raspberry, tobacco, swede, wormwood, iris and other plants are the same allopolyploids of hybridogenic origin.
Question 2. Describe the mechanisms of the main ways of speciation.
There are allopatric and sympatric ways of species formation. In allopatric speciation, also called geographical, the obstacles to interbreeding are primarily due to the spatial separation of populations. spatial isolation leads to the division of the species range into separate isolated zones (and, as a result, into isolated populations). If the habitat conditions in these zones are different, new species may gradually form. Genetic isolation develops secondarily. So, once in Australia there was one species of parrots of the genus Rahycephala. In the conditions of the dry period, the single area was divided into the western and eastern zones. Over time, individuals of the two populations acquired morphophysiological differences that made it impossible to interbreed when the area again became common. There was a formation from one ancestral species of two new ones.
In sympatric speciation (ecological speciation), a new species is formed within the range of the original species. From the very beginning, isolation is genetic. This situation is created as a result of polyploidy due to disturbances in the normal course of meiosis during large chromosomal rearrangements or interspecific hybridization.
Allopatric (geographical) speciation occurs slowly and produces species that, as a rule, differ in morphophysiological criteria from the parent species. The sympatric (ecological) path is relatively fast and gives species close to the original in terms of morphophysiological indicators.
Question 3. What role does isolation play in the process of speciation?
The division of a single species into several isolated populations and their further existence under different conditions is the main mechanism of speciation. At the same time, the exchange of genes between populations stops and interpopulation differences gradually accumulate, leading over time to the formation of new species. Differences that change marriage rituals are of particular importance. They prevent interbreeding of individuals belonging to different populations, even if they meet by chance.
Question 4. Give examples of geographical and ecological speciation.
An example of geographic speciation is the appearance of various types of lily of the valley as a result of the division of a single ancient area of the original species by a glacier into several isolated zones. So, once in Australia there was one species of parrots of the genus Rahycephala. In the conditions of the dry period, the single area was divided into the western and eastern zones. Over time, individuals of the two populations acquired morphophysiological differences that made it impossible to interbreed when the area again became common. There was a formation from one ancestral species of two new ones. Another example is the famous Galapagos finches.
As an example of ecological speciation, one can cite various types of oaks that grow on different soils: some on limestone, others on igneous, and still others on black soil. The trout of Lake Sevan are divided into six separate populations, spawning in different rivers and streams that flow into the lake. Ecological isolation can be a consequence of human impact: only those plants that have time to bloom and produce seeds either before haymaking or after are preserved in hay meadows. Thus, two populations are formed from one population: one with early flowering plants, the other with late flowering plants.
Question 5. What is the significance of spatial isolation for the formation of new species?
Spatial isolation is the most important prerequisite that can lead to geographic speciation. Mountain ranges and vast water spaces are most often used as factors of spatial isolation. They can also be deserts, areas with any specific deviations in the composition of the soil (salinization, acidification), etc.
Spatial isolation can also occur in the absence of visible geographic barriers. The reasons for it in this case lie in the limited "radii of individual activity." Thus, in the "coastal" eelpout fish Zoares viiparus, from the mouth at the end of the fjord, the number of vertebrae and rays of some fins decreases. The conservation of variability is explained by the sedentary way of life of the eelpout. Such variability is also observed in mobile species of animals, for example, migratory birds with nesting conservatism. Juvenile swallows, for example, return from wintering to their birthplace and nest within a radius of up to 2 km from the mother's nest. Interbreeding in swallows is limited to a group of closely nesting individuals. In contrast to separation by barriers, this kind of geographic isolation is referred to as separation by distance.
